On The Thermodynamic Representation Of Fragility Of Metallic Glass-forming Liquids

We investigate the relationship between fragility of supercooled liquids and fragility of superheated melts based on thermodynamic representation of metallic glasses. The glass transition temperature, Tg and cluster stability are important parameters for fragility research. We find that glass transition temperatures for bulk metallic glasses have a linear connection with chemical enthalpy,△Hchem. And the chemical enthalpy can reflect excess entropy at glass transition temperature. The linear fitted slope of Tg-|△Hchem| line has a close relationship with fragility of supercooled liquids.Based on the Idealized-Atomic-Packing (IAP) model, we propose an model to descibe the clusters in AI-TM(transition metal element)-RE(rare earth element) amorphous structure-(Al5TM)x和(Al10RE)y, where x and y are composition concentrations of transition metal and rare earth, respectively. The parameter (6x+11y) deduced from comparison of Al concentration for quasi-equivalent structure (Al5TM)x and (Al10RE)y and Al100-x-y TMxREy, can predict crystallization processes, which proves that the quasi-equivalent structure parameter is very effective to reflect actural amorpous structure and is very important for investigation of cluster stability. Al-TM-RE glasses with parameter (6x+11y) less than 100 correspond with nanocrystalline glasses. When (6x+1y) is colse to 110, Al-TM-RE glasses will undergo nanoglassy crystallization behavior. When (6x+11y) values are bigger than 110 and less than 126, Al-TM-RE glasses will undergo glassy crystallization behavior.Al-TM-RE metallic glasses with "6x+lly=100" show smaller fragility of supercooled liquid, while Al-based glasses with "6x+11y=126" show better glass forming aility. Thses results indicate that quasi-equivalent structure is an effective manner in evaluating fragility of supercooled liquids, which is consistent with Adam-Gibbs equation. When (6x+11y) value for Al-TM-RE is near 100, the system will have high degree of short-range-order clusters, and the cluster size is smaller because there are Al atom clusters besides (A15TM) and (Al10RE) quasi-equivalent clusters. And we find glass forming ability can not be described by fragility of surpercooled liquid, but has correlation with the similarity of amorphous structure and eutectic crystalline strcture.We modify the Nearly-Free-Electron approach by IAP model to evaluate stability of metallic glasses. And we find that electron concentration, e/a, and equivalent total atomic number per cluster, N, which can be derived from chemical composotin are important factors for cluster stability and glass forming ability. When N and e/a satisfy the following expression:The solute-centered solvent-surrounded clusters are stable and clusters with larger size have the highest stabiltiy, which is consistent with the research results that fragility of supercooled liquids can not evaluate glass forming ability.The fragility parameter of superheated melts, M is considered to have close correlation with glass forming ability. We find that the smaller value of M can reflect smaller difference in Gibbs free energy between liquid and crystalline states and higher degree of short-range-order clusters in superheated melts. For a given alloy system, smaller M values usually correlate with eutectic compositions. For metallic melts, M values are close to 1.The fragility of supercooled liquids are determined mainly by mixing entropy (Sσ/kB) of alloys. Supercooled liquids with larger mixing entropy value illuminate that the activation barriers for diffusive rearrangement of a rearrangeable region is smaller and tend to become less stable. To correlate the fragility of supercooled liquids with superheated melts in different alloy systems, we propose a parameter (M/(Sσ/kB)) to reflect stability of supercooled melts, which is closely related to glass formation.Based on the influence of similar strucutre between melts and metallic glasses on glass forming ability, we find that (M/(Sσ/kB)) parameter can measure this similarity. According to the similar structure between fcc-Al and Al-Ti-B master alloy, we fabricate new Cu50Pr30Ni10Al9.9Ti0.05B0.05 and Al58.4Cu22.1Pr13.3Ni4.4Ti0.9B0.9 metallic glasses by arc melting under a Ti-gettered argon atmosphere for four times to ensure homogeneity of the samples. And according to our research results on quasi-equivalent strucutre of metallic glasses, we fabrication (Ni-P)-(Fe78Si9B13)-(Ni-P) amorphous composite by electroless plating Ni-P on Fe78Si9B13 amorphous ribbon. Fe78Si9B13 amorphous ribbon acts as catalyst, which makes Ni-P amorphous film grow one-dimensionally and form flat surface.